1. Field of the Invention
The present invention relates to a black and white stretching system in an image processing system for processing an image signal, and more particularly, to a black and white stretching system and method which can adaptively stretch the level of an input image signal to a mean level and can improve the efficiency of removing a flicker phenomenon of a picture by varying a filter coefficient by using histogram information.
2. Description of the Related Art
Generally, according to the image signal process of a conventional image processing system, the distribution of the dynamic range of the level of an input image signal is smaller than that of the hardware range of the image signal process. Typically, resolution is increased through the use of a black and white stretching system. The system stretches the dynamic range of the level of the image signal so that the range is increased to near that of the hardware range. Hereinafter, the term “image signal” refers to a brightness signal, one of among many signals which form a complete image.
FIG. 1 is a schematic block diagram of a conventional black and white stretching system. FIGS. 2A and 2B are views explaining a conventional method of stretching an input image signal performed by the black and white stretching system illustrated in FIG. 1.
The black and white stretching system has a maximum/minimum detection unit 11, a mean calculation unit 13, a stretching point determination unit 15, a stretching function generation unit 17, and a stretching unit 19.
The maximum/minimum detection unit 11 detects the maximum value and the minimum value of an input image signal, i.e., a brightness signal, and the mean calculation unit 13 calculates the mean value of the input image signal.
The stretching point determination unit 15 determines stretching points LB and UW corresponding to a dynamic range of the input image signal by comparing predetermined stretching points L and U with the calculated mean value. Specifically, the lesser of a predetermined stretching point L and the mean value is determined to be the black stretching point LB. The greater of a predetermined stretching point U and the mean value is determined to be the white stretching point UW.
The slope calculation unit 17 calculates a black stretching slope SLB using the detected minimum value, mean value and black stretching point LB, and calculates a white stretching slope SLW using the detected maximum value, mean value and white stretching point UW.
The stretching unit 19 stretches the range of a gray level of the input image signal to the hardware range by mapping the calculated black stretching slope SLB and the white stretching slope SLW onto the input image signal.
FIGS. 2A and 2B are views explaining the stretching of an input image signal performed by the conventional black and white stretching system.
As shown in FIG. 2A, if the mean value of the input image signal lies between the predetermined stretching points L and U, the predetermined stretching points L and U are determined by the stretching point determination unit 15 to be the black and white stretching points LB and UW, respectively. Thus, the improvement of the contrast of the picture can be expected. However, as shown in FIG. 2B, if the mean value of the input image signal is less than the value of the predetermined stretching point L, the mean value is determined to be the black stretching point, and the corresponding black stretching slope SLB is obtained.
Through this system, the image signal mapping on the black stretching slope SLB can obtain an excellent contrast effect in a dark picture, but in a bright picture, the contrast effect is relatively degraded due to the white stretching slope SLW according to the white stretching point UW to which the mean value is not applied.
Accordingly, the conventional stretching system has the problem that the optimum contrast effect cannot be obtained because the stretching slopes are irrespective of the change of the mean value in the dark and bright pictures.
Additionally, the conventional stretching system uses a simple filter (not illustrated) with a fixed filter coefficient in order to remove the flicker phenomenon of the picture generated by the above-described stretching. With a simple filter, if there is any scene change or change of the picture, information of the previous picture is accumulated and reflected in the present picture in the fixed ratio. Thus, the removing efficiency of the flicker phenomenon of the picture deteriorates due to the unnatural change of the picture and the change of the gray level.